In many industrial processes, it is necessary to cut a continuous web material, fed along a feeding path, into single sheets.
This kind of cutting device is typically used in the production of cellulose sheets, for instance sheets of corrugated cardboard.
Corrugated cardboard is produced continuously starting from reels of paper, from which at least three paper sheets are unwound and then glued together. Before being glued, the intermediate sheet is processed by means of a corrugator to form transverse flutes that are glued to the two outer sheets, or webs, forming the so-called liners of the finished corrugated cardboard. The corrugated cardboard can also comprise more than three sheets glued together, arranging corrugated sheets between smooth sheets. The web of corrugated cardboard is transversally cut into individual sheets by means of a so-called shearing machine. The shearing machine usually comprises two opposite cutting devices, each of which includes a roller and a respective blade applied thereto. The two rollers rotate synchronously and the two blades cut the corrugated cardboard into individual sheets, whose length can be defined according to production needs. By changing the speed of rotation of the two rollers of the cutting devices forming the shearing machine, it is possible to produce, in quick succession, individual batches of sheets of different dimensions.
Usually, the roller of each cutting device is hollow and supported onto a stationary shaft, this latter being fastened to two flanks. The stationary shaft comprises a plurality of rolling bearings supporting the hollow roller, onto which the blade is applied, forming the cutting member of the device. In some embodiments, the blade can be parallel to the axis of rotation of the roller. The blade is preferably inclined with respect to the axis of rotation of the roller, i.e. it has a helical shape, so as to gradually cut the corrugated cardboard sheet. In this case, the axes of rotation of the two opposite rollers of the cutting devices forming the shear machine are inclined, with respect to the feeding direction of the web material to be cut, by an angle other than 90°, in order that the cut done by the inclined blades is orthogonal to the feeding direction of the web material.
The cutting rollers are hollow and supported onto a stationary shaft because they are subject to significant acceleration and deceleration dynamic stresses, as they work at a non-constant speed of rotation and can remain stationary for given time intervals. Namely, the cutting rollers rotate at constant speed only when the sheets to be cut have a longitudinal dimension (i.e. in the feeding direction of the cardboard along the feeding path) equal to the circumferential extension of the cutting rollers. In all other cases, i.e. when this dimension is lower or greater than the circumferential extension of the cutting rollers, these latter shall be cyclically accelerated/decelerated in order to synchronize the cutting operations with the feeding movement of the web material, so as to have sheets of the desired length.
More in detail, if the sheets to be cut have a longitudinal dimension greater than the circumferential extension of the cutting rollers, these latter shall slow down or, in some cases, even stop between a cut and the following one. Vice versa, if the sheets to be cut have a longitudinal dimension lower than the circumferential extension of the cutting rollers, these latter shall accelerate between a cut and the following one. Actually, the cut shall be done at a peripheral speed of the cutting rollers, and of the respective blades, equal to the feeding speed of the web material, i.e. of the corrugated cardboard; otherwise, the corrugated cardboard would be damaged by the blades and cut not orthogonally.
The corrugated cardboard is fed along the feeding path at relatively high speed, in the order of 200-400 m/min, for example, and for this reason the accelerations/decelerations to which the cutting rollers are subject are highly significant. It is therefore necessary that the cutting rollers have low inertia; for this reason they are hollow, in order to reduce the moving mass as much as possible.
Cutting the cardboard causes dynamical loads onto the blades, and thus onto the hollow rollers, that are transmitted i onto the respective stationary shaft through the rolling bearings supporting the hollow roller.
In order to accurately cut the sheets and achieve a good quality thereof, it is necessary to reduce the deformations resulting from the dynamic stresses onto the blades, the hollow rollers and the shafts. These deformations can be reduced by increasing the diameter of the stationary shaft and, thus, the diameter of the hollow rollers supported thereon.
There are shear machines provided with roller bearings interposed between the stationary shaft and the hollow cutting roller of each cutting device so as to achieve a greater stiffness. However, due to the sizes of the bearings interposed between stationary shaft and hollow roller, the shaft has a diameter substantially smaller than the diameter of the hollow roller.
There are machines that use cams acting between the hollow roller and the inner stationary shaft in order to adjust, to a certain extent, the deformation to which the shaft and the roller are subject during the cut. This adjustment can be modified only by regulating the cam system again, and this is quite laborious. Moreover, the use of adjustment cams does not limit possible vibrations.
Moreover, the existing shear machines are subject to thermal expansions resulting from the non-uniform heating of the cutting roller, and these thermal expansions can cause cut inaccuracies. The expansions depend on the operating conditions.
There is therefore the need for a cutting device and a shear machine comprising this cutting device, which completely or partly overcome or alleviate the above mentioned drawbacks.